Note 3 — Chapter IX · THE NORTH STEAMING ERROR
Reading edition
The value of the latitude error is b tan L, where L is the angle of latitude and b a constant dependent upon the design of the compass. In the Sperry correction mechanism the value of the quantity b is represented by the distance between the centre of the dial B and the centre of the pin C when the radial slot in the dial is lying at right angles to the bar D E.